The present invention relates to interference cancellation, and more particularly, to techniques for canceling periodic interference from sampled data.
Data sampled by an electronic device may include noise from the source supplying power to the device. In order to use the data, the noise often must first be removed.
An electrocardiograph or ECG machine is one example of a device that can be susceptible to power supply interference. Electrocardiographs are generally powered by standard electric power, which can be delivered as a sinusoid, at a frequency of sixty-hertz in the United States and Canada and fifty-hertz in Europe and other countries. While power is ideally delivered at a frequency of 50 or 60 Hz (the xe2x80x9cnominal frequencyxe2x80x9d), the frequency may also vary from between approximately 40 and 60 Hz, depending upon load and other factors. It is within this frequency range that the power source noise can interfere with electrocardiograph operation.
Known devices that mitigate power supply interference, such as power source filters, for example, are typically set to filter everything but the frequency of interest, and are not capable of learning and eliminating power supply interference if the frequency of interference is different from the frequency setting of the filter. This inability to filter an interference signal having a changing frequency can pose significant risks when treating heart conditions and heart failures.
While the invention is described below in the context of an electrocardiograph, it should be understood that the invention is equally applicable to any electronic device that samples data.
The invention mitigates the above-described problems by providing a processing apparatus and a method that removes a periodic interference signal, even if it has a changing frequency or a frequency different from the nominal frequency, from a desired sampled signal. The invention removes signal interference through frequency hunting and signal separation.
More specifically, the present invention provides an apparatus for and a method of conditioning a signal, which signal includes a desired signal or xe2x80x9csignal of interestxe2x80x9d overlapped or combined with an interference signal of unknown frequency, amplitude, and phase. Preferably, the interference signal is a periodic signal, such as a sinusoidal signal like a power-line or a power-supply signal, for example.
In one aspect of the invention, the method approximates the frequency and amplitude of the interference signal through reverse direction filtering before separating the interference signal from the input signal. The approximation preferably estimates the frequency and amplitude of the interference signal using a control function or algorithm, such as a xe2x80x9cbangxe2x80x94bangxe2x80x9d algorithm, a xe2x80x9cpiece-wisexe2x80x9d linear (a.k.a. xe2x80x9clinear within limitsxe2x80x9d) control function, or a sigmoid control function.
In another aspect of the invention, the method extracts a sample of the interference signal from a sample of the input signal through signal processing. In one exemplary embodiment, a second signal of suitable approximation in frequency and amplitude to the interference signal is sampled and that sample is then subtracted from an input signal sample to separate that portion of the interference signal that was overlapped or combined with the sample of the desired signal. The second signal frequency and amplitude may be derived from an iterative process that measures the amplitude of the interference signal and the phase angle between the interference signal and an output signal and then adjusts the frequency and gain of the second signal based, in part, on these measurements until the second signal substantially matches the interference signal.
In yet another aspect of the invention, the method removes the interference signal through filtering. In one exemplary embodiment, an active tunable notch filter is used to attenuate the interference signal. By actively identifying and tracking the frequency of the interference signal, the embodiment positions a filter null at the interference signal frequency. Although the exemplary embodiments describes a notch filter, the invention may use one or more filters that are capable of rejecting interfering signals in a frequency range such as on the order of two to twenty hertz, with a twenty hertz frequency range being the preferred range.
The disclosed methods can be implemented through software, analog, or digital circuitry or by any combination of these technologies. Preferably, when a reverse direction learning technique is implemented in at least a partially digital embodiment, the method processes the input signal in a reverse time direction (i.e. processing data from the last sample acquired to the first sample acquired within the data window) in order to learn the frequency, phase and amplitude of the interference signal prior to generating a filtered output. Stated differently, the learning of the frequency, phase and amplitude is done in the reverse direction (i.e., looking backward at already acquired data), while the filtering proceeds in the forward direction.
It is an advantage to provide a method of filtering power line interference from a signal of interest which method is not constrained to signals in which the frequency of the interference signal is constant or known in advance.
It is another advantage to provide a filtering method that is not limited to a specific sampling rate.
These features as well as other advantages of the invention will become apparent upon consideration of the following detailed description and accompanying drawings of the embodiments of the invention described below.